Kj. Collier et Md. Wakelin, INSTREAM HABITAT USE BY BLUE DUCK (HYMENOLAIMUS-MALACORHYNCHOS) IN A NEW-ZEALAND RIVER, Freshwater Biology, 35(2), 1996, pp. 277-287
1. The feeding habitat of a river specialist, blue duck (Hymenolaimus
malacorhynchos (Gmelin 1789): Anatidae), was characterized in terms of
water depth and velocity on eight occasions over a 13-month period in
a river in the central North Island of New Zealand using video to rec
ord activity and relocate feeding sites. 2. Of the five feeding activi
ties identified ('pecking', 'grazing', 'head-dipping', up-ending' and
'diving'), adult blue duck used mostly head-dipping (> 60% of feeding
events on all dates), although diving or grazing from submerged surfac
es of exposed boulders comprised major proportions of feeding behaviou
r (up to 33%) on occasions. Variations in feeding behaviour between da
tes partly reflected changes in antecedent flow conditions and the ann
ual cycle of the birds. 3. Grazing and diving occurred in significantl
y faster water (mostly 0.3-0.45 m s(-1)) and at significantly differen
t depths (mean = 0.10 and 0.55 m, respectively) than head-dipping (0.2
0 m depth and 0.28 m s(-1) velocity). Adult feeding depths and velocit
ies at four sites on different dates averaged 0.20 m and 0.31 m s(-1),
respectively. Most feeding by 3-4-week-old ducklings occurred over a
similar distribution of water velocities to adults but over a wider ra
nge of depths. 4. Adult birds fed in significantly shallower and lower
velocity water than was available on the two dates that comparisons c
ould be made. Ducklings also fed over a slower range of water velociti
es but were not selective in terms of water depth. 5. Energetically mo
re expensive search methods were employed at times of high apparent en
ergy demand to access flow microhabitats where larger bodied prey were
more likely to be encountered. 6. These data indicate that, like othe
r aquatic organisms, river birds can be influenced by basic hydraulic
elements of river flow, but show at the same time that adult blue duck
can accommodate variable lotic environments efficiently.